• Blending theory of inheritance: hereditary trait blend evenly in offspring through mixing of parent’s blood.
  • Mendel studied heritable characteristics called characters; plant color and seed shape.
  • Variation in character= Trait
  • Monohybrid cross between to heterozygous parents (Dr x Dr) F1 generation will yield phenotypic ration of 3:1 (1/4 getting PP or pp, or ½ getting Pp)

Mendel hypothesis:

  • Genes that govern genetic character occur in pairs in individuals (chromosomes carry genes; different versions of genes= alleles)
  • If different alleles are present in an individual’s pair of genes, one allele is dominant over the other. The dominant allele is always expressed. Recessive allele only expressed when two of them exist.
  • Pair of alleles that control a particular character segregate as gametes are formed; enter gamates singly; half into each= Principle of Segregation


  • Probability- how Mendel determined the proportions of offspring that would be produced
  • Product-Rule. If two events happen independent of each other; you multiple them together (i.e rolling a double 4, (1/6) of getting 4 on one die, (1/6) of getting 4 on the other die (total probability= 1/36)
  • Sum-Rule. Applies when different events all give the same outcome; you add them together (i.e chance of rolling a 7, (1&6, 2&5, 3&4) each happen independently, so each has (1/36) add them up, 6 times. 1/6!!!
  • Punnett Sqaure: method for determining genotypes of offspring and their proportions.
  • Testcross: cross between individual with unknown genotype, but dominant phenotype with homozygous recessive individual, called testcross.

  • Dihybrid: zygote produced from cross with two characteristics.
  • Dihybrid cross: 9:3:3:1 ratio (each has a 1/16 chance of occurring together)

  • Principle of Independent Assortment: alleles of genes that govern two characters segregate independently during formation of gametes. (R or r) govern seed shape, DO NOT influence where (Y or Y) govern seed colour go.


  • Chromosomes occur in pairs in sexual reproduction, like alleles
  • Chromosomes each segregated singly in gametes, like alleles
  • Chromosomes segregate independently of each of other pairs, like alleles
  • Each sister chromatid in a chromosome pair, is derived from maternal/ paternal source, like alleles

Incomplete dominance: occurs when effect of recessive alleles can be detected in heterzygotes. (i.e snapdragons; mixing red and white purebleed, gets pink. CRCW. 1:2:1; one allele is not completely dominate over the other. CR makes red pigment, CW, makes none; so when mixed, can only make enough pigment to make pink flower.

i.e sickle cell disease; can be homozygous recessive, can’t make the heme group at all, or heterozygous recessive; only code for half; milder form.

Many alleles when analyzed with biochemical or molecular level appear to be actually incomplete dominant.

Codominance: alleles are approximately equal effect in individuals; alleles are equally detectable in individuals. Examples of this is blood types; ABO, iAiB.

Each genotype has different phenotype; inheritance pattern for blood group alleles is generally the same for incomplete dominance.


Multiple alleles: more than two different alleles of gene if entire population is considered. Each person will only have two (ba/bc) but there could be much more alternative (ba/bb/bc/bd/be)

Person with blood type A has antigen A, antibody B

Epistatis: gene interacts with one or more alleles of a gene at one locus, inhibiting; masking alleles of a gene in a different locus RESULT: expected phenotypes do not appear among offspring.

i.e retrievers; BB= black fur, bb= chocolate fur. E determines whether the pigment is even deposited (EE, yes) (ee= no) gene E is epistatic to gene B.

Polygenic inheritance: several gene contribute to same character; causes continuous distribution of gradations. Quantitative traits: skin colour, weight, height; all undertake similar continuous distribution.

Polygenics inheritance, detected by defining classes of variation; 180cm, 181, put into bell-shaped curve graph; few extremes at each end.

Pleiotropy: single gene affects more than one character of an organism. Alteration in single gene sometimes affects more than one phenotype in an organism.

author avatar
William Anderson (Schoolworkhelper Editorial Team)
William completed his Bachelor of Science and Master of Arts in 2013. He current serves as a lecturer, tutor and freelance writer. In his spare time, he enjoys reading, walking his dog and parasailing. Article last reviewed: 2022 | St. Rosemary Institution © 2010-2024 | Creative Commons 4.0

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